I'd like to follow the procedures outlined in o2ukjailbreak to enable edge and visual voicemail on my pay as you go SIM. This seems to involve changing some files on the iPhone. How could this be reversed? Because eventually will want to use my T Mobile contract SIM. Or could the settings easily be changed again for T Mobile?

Feeling adventurous, I went ahead without further ado and followed the o2ukjailbreak instructions. I had already activated and jailbroke my 1.1.2 UK phone, and it's all working, on 1.1.2.

I entered these commands:

rm /var/root/Library/Preferences/com.apple.carrier.plist

ln -s /System/Library/Frameworks/CoreTelephony.framework/Support/O2_UK.plist /var/root/Library/Preferences/com.apple.carrier.plist

rm -f /var/root/Library/Preferences/SystemConfiguration/preferences.plist

Then re-booted. The first visible change is that the E (EDGE) symbol has been replaced by a solid block.
When trying to connect to the internet via Safari, I get the message "Could not activate EDGE: You are not subscribed to EDGE". As far as I know, I am though (I use an O2 Pay as you go, with unlimited Internet bolt on).
Also, the visual voicemail does not work; I can only access my voicemail the normal way. I don't even know how visual voicemail looks like, of course.

So I guess something has gone wrong somewhere. I'd be ever so grateful for any hints!